13. An introduction to AS Level organic chemistry

In the syllabus, X represents a halogen atom, while R and R′ represent an alkyl group or hydrogen where appropriate. These symbols show the reaction pattern without committing to one named molecule.

Read the notation together with the functional group: R–X is a halogenoalkane, R–OH an alcohol and R–COOH a carboxylic acid. R and R′ need not be identical, and X is not a variable element chosen after the equation.

The substitution pattern R–Br + OH⁻ → R–OH + Br⁻ describes many bromoalkanes. A specific example such as CH₃CH₂Br is one member of the general family.

Do not treat R as “any atom” or replace X by a whole molecule. The symbols have fixed structural meanings.

A functional group is the atom or bond arrangement that gives an organic molecule its characteristic reactions. At AS level, recognise alkenes, halogenoalkanes, alcohols, aldehydes, ketones, carboxylic acids, esters and amines from their structures.

Identify the group before choosing a reagent or mechanism. For example, C=C suggests addition chemistry, C–X suggests substitution or elimination, and C=O distinguishes carbonyl compounds from hydrocarbons.

CH₃CH₂OH is an alcohol because it contains –OH; CH₃CHO is an aldehyde because the carbonyl carbon is bonded to H. The names follow the structure, not the other way round.

A functional group is not just a substring in a name. Check connectivity and distinguish an aldehyde, ketone, acid and ester carbonyl.

A hydrocarbon is a compound made only from carbon and hydrogen atoms. Alkanes, alkenes and alkynes are hydrocarbons; adding oxygen, nitrogen or a halogen makes a different class of organic compound.

The definition is about composition, not whether the molecule is saturated or unsaturated. Use the molecular formula and displayed structure to check the atom types.

C₃H₈ and C₃H₆ are both hydrocarbons, while C₃H₇OH is not because it contains oxygen. The first two can belong to different homologous series.

“Organic” is broader than “hydrocarbon”. Many organic compounds contain heteroatoms and are still organic.

Alkanes are hydrocarbons containing only single C–C and C–H bonds. They are saturated because each carbon has the maximum number of hydrogen atoms allowed by four covalent bonds.

The absence of a reactive functional group helps explain their relatively limited chemistry: combustion, free-radical substitution and cracking are the key syllabus reactions. Their single bonds are still covalent bonds, not “no bonding”.

Propane, CH₃CH₂CH₃, is an alkane. It can burn in oxygen or undergo chlorination under radical conditions, but it does not decolourise aqueous bromine as an alkene does.

Saturated does not mean unreactive under every condition, and an alkane is not defined by its physical state.

A functional group is the structural feature that controls a family’s typical reactions and many of its physical properties. The carbon skeleton still affects boiling point and steric access.

Identify the functional group first, then choose a reaction type. For example, an alkene C=C undergoes electrophilic addition, an alcohol can be oxidised, and a carboxylic acid can form an ester.

Ethanol and ethanoic acid both contain oxygen but behave differently because –OH and –COOH are different functional groups. Their names and reactions should not be inferred from the element list alone.

A molecule can contain more than one functional group. Do not assume one heteroatom automatically defines the whole reaction.

A general formula describes a homologous series, a structural formula shows how atoms are connected, a displayed formula shows all bonds, and a skeletal formula abbreviates the carbon framework.

Convert between representations without changing connectivity. In skeletal formulae, every line end and vertex is a carbon unless labelled otherwise, and hydrogens on carbon are implied to complete valency four.

Propene can be written C₃H₆, CH₃CH=CH₂, a displayed structure or a zig-zag skeletal drawing. All four representations describe the same molecule.

A shorter skeletal drawing is not a different compound. Check the implied carbon and hydrogen counts before naming or calculating formulae.

Systematic naming starts with the longest continuous carbon chain containing the principal functional group. Number it to give the suffix and multiple bonds the lowest possible locant, then add substituents.

Use the syllabus scope: straight-chain examples up to six carbons, with the stated limits for esters and nitriles. The suffix identifies the main functional group; prefixes describe substituents or halogens.

CH₃CH₂CH₂OH is propan-1-ol, while CH₃CH(OH)CH₃ is propan-2-ol. The number changes because the hydroxyl group is in a different position.

Do not number from the nearest end without checking the principal group, and do not treat a branch as part of the parent chain automatically.

The molecular formula counts every atom in the displayed or skeletal structure. The empirical formula is the simplest whole-number ratio of those atoms.

Count carbons and heteroatoms directly; infer hydrogens from the bonds and carbon valency. To obtain the empirical formula, divide all subscripts by their greatest common factor.

Butan-2-ol has molecular formula C₄H₁₀O. Because the subscripts have no common factor, its empirical formula is also C₄H₁₀O; C₆H₁₂ would reduce to CH₂.

Do not simplify a molecular formula when the question asks for it, and do not count the line segments in a skeletal formula as atoms.

A homologous series shares a functional group and general formula, with successive members differing by CH₂. Saturated compounds contain no carbon–carbon multiple bond; unsaturated compounds do.

Homolytic fission splits a bond evenly to form radicals, while heterolytic fission gives both electrons to one atom and forms ions. In radical chemistry, initiation creates radicals, propagation consumes and regenerates them, and termination removes radicals.

UV light can split Cl₂ homolytically: Cl₂ → 2Cl·. A chlorine radical then abstracts H from methane, beginning a propagation chain.

A radical is not simply an ion, and “unsaturated” does not mean every bond is weak. Match the term to the electron movement shown.

Free-radical substitution replaces a hydrogen by a radical; electrophilic addition adds an electrophile across a C=C bond; nucleophilic substitution replaces a leaving group; nucleophilic addition attacks a polar carbonyl.

Curly arrows show electron-pair movement and begin at a bond or lone pair. The mechanism follows the species present, not merely the word “organic”.

In bromoethane hydrolysis, an OH⁻ lone pair attacks the carbon attached to Br while the C–Br pair leaves to Br⁻: this is nucleophilic substitution.

A curly arrow is not a reaction arrow and must not start at a positive atom. It represents a pair of electrons moving from a bond or lone pair.

A carbon skeleton can be straight-chain, branched or cyclic. This describes connectivity; it is separate from whether the molecule is saturated and from the functional group it carries.

Trace the carbon–carbon framework before naming a molecule or counting isomers. A ring closes the chain, while a branch creates a carbon substituent attached to the parent chain.

Butane is straight-chain, 2-methylpropane is branched, and cyclohexane is cyclic. All three are hydrocarbons, but their connectivity and physical properties differ.

A cyclic molecule is not automatically aromatic, and a branched molecule does not have fewer carbon atoms than its unbranched isomer.

An sp-hybridised atom has two electron domains and a linear arrangement near 180°. sp² gives three domains and a trigonal-planar arrangement near 120°, while sp³ gives four domains and a tetrahedral arrangement near 109.5°.

These are idealised local geometries. Lone pairs and substituents can distort angles, and the hybridisation label refers to the atom’s bonding environment rather than the whole molecule’s shape.

The carbon atoms in ethyne are sp and linear; each alkene carbon in ethene is sp² and trigonal planar; methane carbon is sp³ and tetrahedral.

Do not call every molecule with one sp³ atom tetrahedral overall. Apply the geometry around the specific atom being considered.

A sigma (σ) bond forms by end-on overlap along the internuclear axis. A pi (π) bond forms by sideways overlap of parallel p orbitals and exists in addition to a σ bond.

A single bond is one σ bond; a double bond is one σ plus one π; a triple bond is one σ plus two π bonds. Hybridisation describes which orbitals make the σ framework and which unhybridised p orbitals make π bonds.

Ethene has five σ bonds and one π bond. Ethyne has three σ bonds and two π bonds, so rotation around the C≡C axis is not a simple single-bond rotation.

A double bond is not “two π bonds”, and a π bond is not a second atom-to-atom connection independent of the σ bond.

A planar arrangement places the stated atoms in the same geometric plane. In ethene, each carbon is sp² and the carbon atoms and attached substituents are approximately planar.

Planarity follows from the local orbital arrangement and restricted rotation around a C=C bond. It is a structural description, not a claim that every atom in a large molecule lies in one plane.

The two carbon atoms and four hydrogens of ethene form a planar group. Replacing the C=C by a single bond changes the rotational freedom and may remove that planar constraint.

Planar does not mean flat in every dimension of the whole molecule, and it does not by itself prove aromaticity.

Structural isomers have the same molecular formula but different atom connectivity. Chain isomerism changes the carbon skeleton, positional isomerism changes a group or multiple bond position, and functional-group isomerism changes the functional group.

Draw the connectivity before comparing names. The atoms and molecular formula stay constant, but the arrangement changes reactivity and often physical properties.

C₄H₁₀ gives butane and 2-methylpropane (chain isomers). C₃H₈O gives propan-1-ol and propan-2-ol (positional isomers), and also methoxyethane (functional-group isomer).

Different conformations from rotation around a single bond are not automatically different structural isomers.

Stereoisomers have the same atoms joined in the same order but differ in spatial arrangement. Geometrical isomerism occurs when rotation is restricted, while optical isomerism arises from a chiral centre and produces non-superimposable mirror images.

A C=C can give cis/trans forms only when each carbon has two different groups. A tetrahedral carbon is a chiral centre when it is bonded to four different groups.

cis-but-2-ene and trans-but-2-ene are geometrical stereoisomers. A molecule with one carbon attached to H, OH, CH₃ and CH₂CH₃ has an optical stereocentre.

Same molecular formula alone is insufficient. Check connectivity first, then test the structural condition for cis/trans or chirality.

Geometrical isomers have the same connectivity but different groups on either side of a double bond. A π bond requires sideways p-orbital overlap, so rotation would break that overlap and is restricted.

Each carbon of the C=C must have two different groups for cis/trans isomerism. “Cis” places corresponding groups on the same side; “trans” places them on opposite sides.

But-2-ene exists as cis-but-2-ene and trans-but-2-ene. But-1-ene does not give cis/trans forms because one double-bond carbon has two H atoms.

A single bond can rotate, so do not infer geometrical isomerism from any pair of substituents without checking the double-bond condition.

A chiral centre is usually a tetrahedral carbon attached to four different groups. Its mirror-image arrangement cannot be superimposed on the original, giving a pair of optical isomers called enantiomers.

Enantiomers have the same connectivity and most achiral physical properties, but rotate plane-polarised light in opposite directions and can behave differently with chiral environments.

In 2-butanol, the carbon bearing OH is attached to H, OH, CH₃ and CH₂CH₃, so it is chiral and gives two enantiomers.

A carbon with two identical groups is not chiral. Do not add advanced meso or diastereomer terminology when the syllabus does not require it.

To find a chiral centre, inspect each tetrahedral carbon for four different attached groups. To find geometrical isomerism, inspect each C=C—or suitable ring—for restricted rotation and two different groups at each relevant position.

Compare the paths around a ring as well as the atoms directly attached. A drawing can hide equivalence, so trace each substituent until the first point of difference.

A substituted cycloalkane can have cis and trans forms even without a C=C because the ring restricts rotation. A carbon with two identical ring paths is not a chiral centre.

Do not count every wedge/dash bond as a chiral centre, and do not call a double bond geometrically isomeric when one end has identical substituents.

To deduce isomers from a molecular formula, first enumerate different carbon skeletons, then place functional groups or multiple bonds, and finally check for geometrical or optical stereoisomers.

Reject duplicates by drawing the same connectivity in a different orientation. Check valency, the required degree of unsaturation and the syllabus limits before counting a structure.

For C₄H₈, consider butenes and methylpropene, then ask whether but-2-ene has cis/trans forms. A cyclic structure may also satisfy the formula if rings are in scope.

Rotating or redrawing one molecule does not create a new isomer. Count distinct connectivity and spatial arrangements only.